Phase modulation



May 11, 1943. l 1 J, VEWWS 2,318,934-

PHASE MODULATIQN y Filed Oct. l5, 1941- wam 1 John, Evans Patented May 1l, 1943 PHASE MoDULA'rroN John Evans, Palmyra, N. J., assignor to Radio Corporation of America, a coporationof Delal ware i Application` October 15, .1941, Serial No. 415,021

6 Claims. My invention relates to radio transmitters and particularly to transmitters for producing a phase modulated or frequency modulated carrier wave. An object of the invention-is to provide an improved method of and means for producing 'a phase modulated or frequency modulated carrier wave.

A further object of the invention is to provide an improved circuit of the push-pull type for phase or frequency modulating the output of a stable oscillator such as a crystal oscillator.

In practicing a preferred embodiment of the -invention, a crystal oscillator operating at the carrier wave frequency has its output circuitconnected to a pair of radio frequency amplifier tubes in push-pull relation. The oscillatoroutput is fed to one of the amplifiersl through a capacitive circuit and to the other amplifier through an inductive circuit. These circuits include ampliiier tubes to which the modulating `signal is applied for varying their plate impedances.v Since the input circuits of the radio frequency amplifier tubes are across these plate impedances, the phase of the radio frequency denser CI to thecontrol grid of tube II, and its lower end is connected through a blocking condenser 23 and a variable inductance coil LI to the control grid of tube I2. The impedance units C I and LI are adjusted to'make their reactances of the same magnitude.

Amplifier tubes II and I2 preferably are so biased that they take very little or no grid current when carrier wave signal is applied to their control grids. With this adjustment, tubes II and I2 operate as class B linear amplifiers. The bias for the tubes Il and I2 may be provided by cathode resistors 24 and 26, respectively, and

supplied to the control grids of tubes through choke'co'ils 2'I and 28, respectively. The cathode resistors 24 and 26 are shunted by the usual bypass condensers indicated at 29 and 3 I.

-The plate impedance of the amplifier tube I 3 is c'onnected across the input circuit of the R. F.

. amplifier II, this connection being from the control grid of tube I I, through a blocking condenser 32 to the plate of tube I3, and through the plate- 1 cathode impedance to ground. A suitable D. C.

tube I3 the R. F. amplifier I2, this connection being Figure 3 is avector diagram which is referred to in explaining the invention.

Corresponding parts in the several figures are indicated by like reference numerals.

Referring to Fig. 1, there is shown a radio' transmitter comprising a crystal controlled 'osciliator I0, a pair of radio frequency amplifier tub II and I2 and a pair of vacuum tubes I3 and I4 to which the modulating signal is applied. The oscillator I8 comprises a vacuum tube I6, a *piezoelectricU crystal I1 in the grid circuit of tube I6. and a tuned,plate circuit which includes from the control grid of tube I2, through a blocking condenser 34 to the plate of tube I4, and through the plate-cathode impedance to ground. A suitable plate 'voltage is applied to the plate of tube Il through a choke coil 36.

The modulating signal is applied to the grids of tubes I3 and Il through a transformer. 31, the secondary of which has its midpoint connected to ground through a-biasing source such as a battery A35. Thus, the modulating signal increases the plate impedance of one tube at'the same time that it decreases the plate impedance 'I of the other tube. It will be apparent that'cur rent at the .oscillator frequency will iiow through the condenser CI and the plate impedance of tubev 4the input circuit of thea. F. ampnner lz. The

tubes I3 and Il preferably .are so biased that substantially no plate current iiows in these tubes when there is no modulating signal being applied through the transformer 31. Therefore, the R. F. amplifier tubes II and I2 continue to function as normal class B amplier tubes when no modulating signal is being applied.

The output circuits of the R. F. amplifiers II and I2 include tuned plate circuits 4I and 42, respectively, coupled to secondary coils 43 and 44. The modulated carrier wave is fed from secondaries 43 and 44 to output conductors 46 through impedance matching transformers 41 and 48 which have their secondaries connected to the conductors 46 in parallel. The output circuits of amplifiers II and I2 may be connected in series or in push-pull to combine the amplifier outputs, if preferred. The conductors 46 may lead to R. F. amplifiers or frequency multipliers.

The D. C. plate voltage for tubes II and I2 may be applied through choke coils 49 and 5I, respectively, which are bypassed by suitable bypass condensers 52 and 53.

Since the amplifier tubes II and I2 are triodes in the particular embodiment illustrated, suitable neutralization is provided for them by means of neutralizing condensers 54 and 56, respectively, these condensers being connected between the control grids of said tubes and points on the tuned plate circuits where the R. F. voltage is 180 degrees out of phase with respect to the R. F.

. tube is decreased. There is no change at the tube I4 since it originally was biased to plate current cut-oir and the grid has now simply been made more negative. Through the plate-cathode impedance of the tube I3, however, there is an increase in the flow of R. F. current i. Since the R. F. voltage E appearing across the tuned circuit 2I--22 is of constant amplitude, the result is as follows:

The increased current flow causes an increase in the voltage drop Eo across the condenser CI, At the same time, there is a decrease in the voltage drop En across the plate-cathode impedance of the tube I3 due to the decrease in said impedance, the latter decrease being more than the current increase. Thus, the R. F. voltage En at the grid of R. F. amplifier II leads the voltage VE appearing across tuned circuit 2I-22 by an amount that increases as the plate impedance of the tube I3 is decreased. This portion of the circuit will produce a leading phase shift of nearly 90 degrees.

Assume now that the modulating signal is positive at the grid of tube I4. Now the tube I3 is at plate current cut-off and the phase shift is produced by current flow i through coil LI across which appears voltage Enand through the tube I4. In this portion of the circuit, the voltage drop En across the plate-cathode impedance of' tube I4 (which is the voltage at the grid of tube I2) lags the voltage E across tuned circuit 2I-22 by an amount depending upon the said platecathode impedance. The more this impedance is decreased, the more the voltage at the grid of R. F. amplifier I2 lags 'the voltage across circuit 2I22. This portion of the circutdwill produce a lagging phase shift of nearly 9 egrees.

From the foregoing. it will be apparent that I have provided an improved way of shifting the phase of a stable oscillator output nearly 180 degrees, that is plus or minus degrees.

In actual practice, the amount of phase shift probably should be limitedto about plus or minus 45 degrees because of the burden otherwise put on the limiter tubes employed for removing the amplitude modulation caused by the change in length of vector Ea (Fig. 3) as the phase is shifted, and, also, because of audio signal distortion that might otherwise become too great. It may be noted that, if a maximum phase shift of about plus or minus 25 degrees is produced (a total phase shift of 50 degrees), the distortion will be negligible, that is, less than two percent, for example.

It will be understood that a suitable correcting network for the modulating signal may precede the transformer 31 to amplify the signal inversely with the frequency to produce a frequency modulated output as is'well known in the art.

I claim as my invention:

l. In combination, a stable oscillator for producing a carrier wave, a pair of ampliers, means for coupling said oscillator to the input circuits of said amplifiers in push-pull relation, means vfor feeding the outputs of said amplifiers to a signal signal is applied from said oscillator to the input circuit of the other of said amplifiers, and means for varying alternately the phase shift produced by said networksin accordance with a modulating signal.

2. In combination, a stable oscillator for producing a carrier wave, a -pair of amplifiers, means for coupling said oscillator to the input circuits of said amplifiers in push-pull relation, means for feeding the outputs of said amplifiers to a signal transfer circuit for combining the outputs,

. said coupling between said oscillator and said input circuits including a -phase advancing network through which signal is applied from said oscillator to the input circuit of one of said amplifiers,

'and a phase retarding network through which signal is applied from said oscillator to the input circuit of the other of said amplifiers, each of said networks including an electron discharge device having a cathode and control grid,l and means including a push-pull connection to said devices for dierentially controlling the impedances thereof in accordance with a modulating signal, and means for biasing the control grids of said devices negative relative to the respective cathodes of said devices by an amount such that substantially no plate current ows in said devices in the absence of modulating signals.

3. In a signalling system, a source of oscillations of carrier wave frequency, a pair of amplifier tubes each having input electrodes, means for coupling said oscillator to the input electrodes of said amplifiers in push-pull relation,

means for feeding the outputs of said amplifiers to a signal transfer circuit for combining the outputs, a pair of modulating vacuum tubes biased to plate current cut-off and having input electrodes coupled in push-pull relation to a source of modulating signal, a phase advancing network in said iirsi-l coupling through which signal is applied from said oscillator to the input electrades of one of said amplifiers. and a phase ntarding network in said rst coupling through which signal is applied from said oscillator to the input electrodes of the other of said amplifiers, one of said networks including one of sad whereby the outputs of said ampliers are fed to said signal transfer circuit for combining the outputs, a pair of modulating vacuum tubes having input circuits connected in push-pull relation to a source of modulating signal, said modulating tubes being biased to plate current cut-off when no modulating signal islbeing applied, connections coupling the plate-cathode impedance of said excitation circuit in series with the other of said plate-cathode impedances.. p y

5. In a, wave length modulation system, a

'source ot wave energy thewave length of which f is to be modulated, a source of modulating signals, a .pair of electron discharge devices each having input electrodes yandhaving output electrodes coupled to an output circuit, a pair of modulator tubes each having electrodes between which there is impedance and an impedance control electrode, a phase advancing network and a. phase retarding network coupling the input electrodes of said devices in push-pull relation to said source of wave energy, said networks .each including the-impedance of a. diierent one of said tubes, a differential coupling between said source of modulating. signals and said tubes, and means for impressing on the impedance control e1ectrode of each tube' ,a bias such that substancoupled to the output electrodes of said amplifiers tially no plate currenttlows inthe said tubes in the absence of modulating signals.

6. In a signalling system, 'a stable oscillator for producing a carrier'vav'e, a pair of ampliers each having an 4input and an output, connections coupling said oscillator to the inputs of said amplifiers in push-pull relation, a signal transferV circuit coupled to the outputs of said. ampliers, a pair of modulating vacuum tubes having inputs connected in push-pull relation to a source of modulating signal, said amplifier tubes each having a plate and cathode, connections coupling the plate-cathode impedance of one of said modulating tubes across the linput of one of said ampliners, said last named connections also Dcoupling.

said plate-cathode impedance of' said one tube in series with one of said first named connections. connections coupling the plate-cathode -impedance of the other of said modulating tubes across the input of the other of said amplifiers, said last named connections also coupling said plate-cathode impedance of said other tube in series with 'another of said first named connections, a capacitive reactance unit in the series connection of said one of said plate-cathode impedances and said one of said first connections, and an inductive reactance unit in the series connection of said other of /said plate-cathode im and said other of said rst named connections.

JonN EVANS.v 

